Abstract: We present infrared JHK photometry of the cataclysmic variable SDSS
J123813.73-033933.0 (SDSS1238)and analyze it along with optical spectroscopy,
demonstrating that the binary system is most probably comprised of a massive
white dwarf with Teff=12000+/-1000 K and a brown dwarf of spectral type L4. The
inferred system parameters suggest that this system may have evolved beyond the
orbital period minimum and is a bounce-back system. SDSS1238 stands out among
CVs by exhibiting the cyclical variability (brightenings). These are not
related to specific orbital phases of the binary system and are fainter than
dwarf novae outbursts, that usually occur on longer timescales. This phenomenon
has not been observed extensively and, thus, is poor understood. The new
time-resolved, multi-longitude photometric observations of SDSS1238 allowed us
to observe two consecutive brightenings and to determine their recurrence time.
The period analysis of all observed brightenings during 2007 suggests a typical
timescale that is close to a period of ~9.3 hours. However, the brightenings
modulation is not strictly periodic, possibly maintaining coherence only on
timescales of several weeks. The characteristic variability with double orbital
frequency that clearly shows up during brightenings is also analyzed. The
Doppler mapping of the system shows the permanent presence of a spiral arm
pattern in the accretion disk. A simple model is presented to demonstrate that
spiral arms in the velocity map appear at the location and phase corresponding
to the 2:1 resonance radius and constitute themselves as a double-humped light
curves. The long-term and short-term variability of this CV is discussed
together with the spiral arm structure of an accretion disk in the context of
observational effects taking place in bounce-back systems.